INTRODUCTION. I9 



avoided and the specimen readily run down to its proper place. Af- 

 ter a little practice, the use of such artificial keys becomes a great 

 pleasure. The benefit has already been referred to. The only secret 

 of their successful use is to be certain that each progressive step is 

 right before it is abandoned for the next. And after a little practice, 

 too, the student will become so familiar with the Algae or with the 

 Desmids that he will not need to apply to the generic keys for aid, 

 but will be able to turn at once to the proper genus, and there use the 

 keys to the species; and with a little further experience he will be en- 

 abled to recognize at a glance the species, and then will be prepared 

 to proceed with his investigations, or to refer intelligently to the ob- 

 ject so as to communicate with his friends and fellow workers in re- 

 gard to it. It is as impossible to speak or to write of an Alga or of a 

 Desmid without using its proper name, as it is to speak or to write of 

 your most intimate friend without using his. 



AlgK and Desmids are singly invisible to the naked eye. It is 

 only when they occur in large masses that the eye can take cogni- 

 zance of them. It rarely occurs, however, that the Desmids are so 

 abundantly congregated that they thus obtrude themselves on the 

 observer. When a large quantity has been collected and the vessel 

 placed near a window, they will collect in a green film at the surface 

 of the water on the lighted side, and there become visible in mass. 

 In the ponds and shallows such an occurrence is not common. At 

 times they are found so abundantly that by holding a glass vessel of 

 the water up to the light they may be seen floating about as minute 

 green objects, which the trained eye will recognize and the pocket 

 lens make distinct. But these varieties are among the largest of the 

 forms; according to my experience they are always exclusively con- 

 fined to the Closteriums. Other large forms, like Micrasterias, at 

 least in the writer's locality, rarely occur in such profusion. To col- 

 lect the Desmids, therefore, it is necessary to collect by faith. The 

 microscopist can know exactly what he has only when he gets home 

 and examines the water drop by drop under the microscope. 



With the Algae it is different. These are usually visible to the 

 naked eye, as they are almost invariably collected in large masses 

 floating on the surface, submerged just beneath the surface, or at- 

 tached in waving tufts or fringes to sticks and stones and other 

 plants in the ponds. The eye of faith is not needed to recognize 

 them. They usually force themselves on the wondering attention of 

 the observing pedestrian in the wayside lanes, beside the ditches and 

 slow brooks. As soft emerald clouds, or graceful streamers floating^ 



